Pytest-vts

Fixed:
- ensure six is a newer version

Fixed:

- update the patch for responses' cookie parsing library.

- supports latest responses

- properly keep strings as six.text_types in the cassete

- should fix using `requests.post(url, json=dict(...))`

Fixed:
- custom wrappers now work properly in playback mode

Added:
- allow more customizations for naming cassettes (based on
callables), although for now the old convention of naming
cassettes is still being the default one.

- allow for custome wrappers around HTTP transactions mocked by vts

Changed:
- reseting the vts recorder now waits for any background jobs
started by the tested code

Removed:

Fixed:
- to be filled in

Fixed:
- proper comparison for warnings when comparing request body vs recorded tracks.

Changed:
- proper comparison for request bodies when matching against recorded tracks.
- unittests vs python 2.7 and 3.6